3D-9C seismic survey for direct S-wave processing and interpretation

Abstract

Conventional P-wave seismic data showed widespread gas clouds in Sanhu area from Qaidam Basin, west China, which made it difficult to map out the subsurface structure because the P-wave is strongly attenuated by the gas clouds. This leads us to use 3D-9C seismic survey to obtain the direct S- wave seismic image by using a S-wave source, which is believed to be little affected by the gas clouds and produce clearer subsurface structure. By using a powerful S-wave vibrator, we can utilize most conventional P-wave seismic data processing techniques to process the direct S-wave data, except the shear wave splitting. Shear wave splitting is an intrinsic characteristic of S-wave propagation through anisotropic media, which splits the incident S-wave into fast (S1) and slow (S2) S-waves. To obtain the reliable split S- wave images from 3D-9C datasets, three kinds of techniques were used: (1) Source correction; (2) radial-transverse (RT) rotation; (3) S-waves splitting. Sanhu area suffers from the strong near surface weathering layer, which results in a thick low-velocity-zone. To address this issue, a ground roll inversion-based surface statistics method was used to improve the quality of S-wave image. The obtained split S- wave images show much clearer subsurface structure, which allows us to study the deposition history and map out the distribution of the channels with lower uncertainty than using P-wave seismic data. The S-wave data is also used for reservoir characterization through S-wave inversion results in collaboration with well-log data.